Isolated point defects possessing a high spin ground state and below-band-gap excitation may play a key role in realizing solid state quantum bits in semiconductors which are the basic building blocks of quantum computers. The silicon vacancy in silicon carbide provides these features, making it a feasible candidate in this special and emerging field of science. However, the exact nature of the luminescence of silicon vacancies detected in hexagonal polytypes has not been clarified. This is the first crucial step needed to understand this basic defect in silicon carbide. We report density functional theory based calculations on the silicon vacancy defect. Based on the obtained results we identify the silicon vacancy related photoluminescence signals with the negatively charged defect.